1. Field of the Invention
This invention relates to a power supply voltage detect circuit. More particularly, this invention relates to a voltage detect circuit for a 3.3/5.0 volt supply voltage in a monolithic integrated audio processing circuit.
2. Brief Description of the Related Technology
The rapid demand for CD audio quality sound within personal computers, PCs, and the transition of power supply levels from 5.0 volts to 3.3 volts has placed compromising decisions on integrated circuit implementations, especially within mixed analog and digital circuits. In the present transitional period, where PCs are changing from 5.0 volt to 3.3 volt operating systems, the requirements on integrated circuits are becoming increasingly more difficult. Manufacturers require that the same chip work in both 3.3 volt and 5.0 volt supply systems, which requires that circuits must operate over a much greater supply range without having any reduction in audio performance. Since integrated circuit devices are being incorporated into both 5.0 volt supply systems (desk tops or work stations) and 3.3 volt systems (lap tops), the need exists for a VCC detect circuit and method for setting analog reference voltages within the audio processing circuitry to compensate for the change from one supply system to the other to maintain the integrity of the audio processing circuitry.
Typically, for 5.0 volt systems, the integrated circuit designer will use smaller physical device sizes to implement integrated circuits to optimize speed and power consumption and to limit substrate noise injection due to rush-through current effects. For a 3.3 volt system, the integrated circuit designer will increase the physical device sizes, or the gate drive, of the digital logic to meet the same timing requirements as required by the 5.0 volt system.
Design tradeoffs for the analog circuitry within the audio processing circuitry are similar to those for the digital circuitry since the designer seeks optimum dynamic range and audio quality performance. Typically, larger full scale reference levels are established for a 5.0 volt system than for a 3.3 volt system. Analog operational amplifiers for audio signals can provide greater voltage swings with a higher power supply level, hence without compensation at the 3.3 volt level, the signal to noise ratio at 5.0 volts is superior to that at 3.3 volts. Compensation at 3.3 volts is necessary to improve the overall audio dynamic range and thus, performance.
The present invention addresses this problem within a mixed analog and digital audio circuitry environment.